Submarine optical cable with water-blocking filling composition

ABSTRACT

Optical cable, in particular for submarine connections. A water blocking resin filling composition of reduced hardness is disposed into interstices. Preferably, the resin composition is a polyurethane resin having less than about 35% by weight of a polyol/polyisocyanate mixture and about 60% to about 90% by weight of a mineral oil. The polyurethane resin preferably has less than about 12% by weight of a coupling agent. The cable has an optical core surrounded by a plurality of metallic wires and an outer polymeric sheath. The optical core is of the “tight” type with a plurality of optical fibers embedded into a polymeric matrix disposed around a strength member.

FIELD OF THE INVENTION

[0001] The present invention relates to a submarine optical cablecomprising a polymeric composition suitable for filling the intersticeswithin said cable, in order to control a longitudinal flow of wateraccidentally penetrated inside said cable.

BACKGROUND ART

[0002] Submarine optical cables are subjected, in case of accidentalrupture thereof, to a sudden ingress of a sea-water flow at highpressure (e.g. 100 bar, when the cable is at 1000 m below the sealevel). Such high pressure water flow may propagate for a relevantlength inside the cable if suitable water blocking means are notprovided in the cable, thus damaging a remarkable portion of said cablewhich has then to be replaced.

[0003] A number of cables designed for submarine installation are knownin the art.

[0004] For instance, U.S. Pat. No. 4,684,213 relates to a submarinecable comprising a pressure resistant steel tube containing opticalfibers, surrounded by two layers of steel wires and by an outer metaltube made of copper or aluminum. Dams of a sticky compound and/or of ajelly of plastic material are disposed at regular intervals inside thecentral tube and in the gaps between the lay of wires disposed betweenthe central tube and the outer tube.

[0005] U.S. Pat. No. 5,125,062 discloses an undersea cable comprising acentral metallic tube, filled with a sealing compound, e.g. silica gel,and containing optical fibers embedded therein, said tube beingsurrounded by a helical lay of metallic (preferably steel) wires.Interstices between wires and between the helical lay and the centraltube are filled with a sealing material, such as a polyurethane resin,which opposes longitudinal propagation of water along the cable.Alternatively, the central tube can be made of plastic and in this casethe helical lay also presents the characteristics of an arch forwithstanding pressure.

[0006] U.S. Pat. No. 4,726,649 relates to a submarine cable wherein thevoids inside the cable are filled with a material having adequateelongation property and creep characteristics, in order to withstand thehigh pressure water flow following an accidental rupture of the cable.As filling material, a polyurethane resin is disclosed, said resin beingcomprised of not less than 10% by weight, preferably not less than 30%by weight of a hydrocarbon polyol/polyisocyanate mixture and from about5 to 90%, preferably from 10% to 70% of a hydrocarbon oil. In theworking examples a paraffinic oil is employed as a mineral oil, in amaximum amount of 56% by weight.

[0007] The applicant has now observed that while prior art resin fillingcompositions, in particular polyurethane based resins, has beendeveloped with selected mechanical characteristics allowing the resin toeffectively block the longitudinal water flow inside a submarine cable,(e.g. following an accidental rupture of the same), no attention hasbeen paid in the prior art cables to the mechanical interaction betweenthe filling compositions and the optical fibers disposed inside theoptical core.

[0008] The Applicant has in fact observed that commercialpolyurethane-based filling composition, when cross-linked, reach arelative high hardness. The Applicant has further observed thatmechanical stresses can be generated in the cable structure, inparticular during the manufacturing process of the cable, such as duringthe stranding of metal armoring wires around the optical core (e.g. as aconsequence of a slight variations in the circular cross-section of thestranded metallic wires). Due to the relatively high hardness of theresin filling material disposed around the optical core, said mechanicalstresses can be transmitted onto the optical core, thus causing apermanent deformation on the structure of the same, with consequentpossible attenuation of the signal transmitted by the optical fibercontained therein.

[0009] Applicant has noticed that this problem becomes much morerelevant when Large Effective Area (LEA) fibers are used in the opticalcable, which fibers are much more sensitive to bending-induced losses(also known as the microbending and macrobending phenomena) thanstandard dispersion-shifted fibers (SDS fibers). The term LEA fibers isintended to encompass those optical fibers having a large effectivearea, in particular optical fibers having an effective area of at least7 μm² or greater. In particular, said LEA fibers may be used inwavelength-division-multiplexing (WDM) and high bandwidth systems.

[0010] The Applicant has now observed that the resin filling material,while being capable of guaranteeing the desired water-blockingperformances, shall have at the same time a relatively reduced hardness,in order not to negatively affect the signal transmission within theoptical fiber in case of mechanical stresses produced onto the cable,and particularly onto the optical core.

[0011] The Applicant has further observed that certain components ofconventional polyurethane resins may negatively interact with somematerial forming the structural elements of the cable. In particular,ester compounds which may be used as plasticizers in conventionalpolyurethane resin composition may negatively interact with thepolymeric material of the optical core of submarine cables. TheApplicant has thus found that by suitably formulating a polyurethaneresin composition, in particular by using a relatively high amount ofmineral oil and a rather limited amount of ester compounds, it ispossible to obtain a good compatibility between the polyurethane resinand other polymeric materials of the cable.

SUMMARY OF THE INVENTION

[0012] One aspect of the present invention thus relates to an opticalcable comprising:

[0013] an outer sheath;

[0014] an optical core disposed within said outer sheath, said opticalcore comprising at least one optical fiber housed therein;

[0015] at least one longitudinal cavity disposed along and in contactwith the optical core; wherein

[0016] said at least one longitudinal cavity is at least partiallyfilled with a resin material having a needle penetration higher thanabout 200 l/10 mm, measured according to ASTM D 5-65.

[0017] Preferably, said resin material has a needle penetration higherthan about 240 l/10 mm.

[0018] Preferably, said resin is a polyurethane-based resin.

[0019] According to a preferred embodiment, said optical core comprisesat least one optical fiber embedded into a matrix of polymeric material.

[0020] According to a preferred aspect, the present invention relates toan optical cable comprising:

[0021] an outer sheath;

[0022] an optical core disposed within said outer sheath, said opticalcore comprising at least one optical fiber housed therein;

[0023] at least one longitudinal cavity disposed along and in contactwith said optical core; wherein

[0024] said at least one longitudinal cavity is at least partiallyfilled with a polyurethane resin obtainable from a liquid curablecomposition comprising less than about 35% by weight of apolyol/polyisocyanate mixture and from about 65% to about 90% by weightof a mineral oil.

[0025] Preferably, the amount of the polyol/polyisocyanate mixture insaid liquid curable composition is from about 10% to about 30% byweight, more preferably from about 15% to about 25%.

[0026] Preferably, said polyol/polyisocyanate mixture comprises fromabout 60% to about 85%, more preferably from about 65% to about 80% of apolyol and from about 15% to about 40%, preferably from about 20% toabout 35% by weight of a polyisocyanate.

[0027] Preferably, the amount of mineral oil in said liquid curablecomposition is from about 65% to about 85%, more preferably from about68% to about 80% by weight.

[0028] Preferably, said liquid curable composition further comprisesless than about 12% by weight of a coupling agent, more preferably lessthan about 8%.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 shows a transversal cross section of a cable according tothe invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] The cable illustrated in FIG. 1 is a submarine optical cablecomprising an optical core 100 surrounded by plurality of metallic wires106 (made of e.g. galvanized steel), said metallic wires being disposedin a helical lay around said optical core and presenting thecharacteristics of an arch for withstanding pressure. Preferably, asshown in FIG. 1, said metallic wires are disposed in two superposedlayers around the optical core.

[0031] A metal sheath 107 (e.g. of copper) is then formed around thehelical lay of metal wires for hermetically protecting the optical corefrom radial ingress of water or humidity.

[0032] An outer polymeric sheath 108, made for instance frompolyethylene, e.g. high or medium density polyethylene, is disposed tosurround the metal sheath.

[0033] In particular, the optical core 100 illustrated in FIG. 1 is ofthe “tight” type, wherein a plurality of optical fibers 103 is embeddedinto a polymeric matrix 102 disposed around a strength member 101. Thestrength member 101 may, for instance, be a wire of steel or resinreinforced with glass fibres, suitable reinforcing polymeric materials(such as aromatic polyamides, for example Kevlar®), carbon fibres or thelike. Examples of polymers suitable for forming the polymeric matrix 102are thermoplastic resins. Preferably an elastomeric polyester is used,such as the one marketed under the trade name Hytrel® by Du Pont, forexample Hytrel® 4056, 3548 L or G3548W. The optical fibers embedded intothe polymeric matrix are conventional optical glass fiber with aconventional polymeric coating, e.g. of UV cured acrylate. Preferablysaid fibers are LEA fibers, which are particularly suitable for thelong-haul communication connections.

[0034] A polymeric sheath 104, e.g of nylon, is preferably disposed tosurround the polymeric matrix 102, in order to avoid the direct contactof this latter with the filling resin material disposed into interstices105. The resin material according to the invention is disposed in theinterstices 105 between the optical core and the metal wires and in theinterstices between said metal wires. The filling of said intersticeswith the resin material can be accomplished for substantially the wholelength of the cable or preferably in a discontinuous manner, i.e. apartial filling. Partial filling refers thus to the fact the filling ofthe interstices is not accomplished for the whole length of the cable.

[0035] However, when a longitudinal portion of an interstice is filledwith a filling material, said filling material will substantiallycompletely fill the whole radial portion of said interstice, in order toavoid the possible formation of preferential longitudinal paths alongwhich water may flow.

[0036] To this end, longitudinal interstices 105 can be filled with aresin material according to the invention for portions of about 20-40meters length, separated by portions of about 10-30 length substantiallyfree from said filling material. For instance, said interstices can befilled for portions of about 30 meters length with a resin materialaccording to the invention, separated by portions of about 20 meterslength free from said material.

[0037] The polymeric resin material used for filling the interstices 105of the cable structure shall be sufficiently soft so as not tonegatively affect the signal transmission of optical fibers in case ofmechanical stresses produced onto the cable. In particular saidpolymeric material has a needle penetration higher than about 200 l/10mm, measured according to ASTM D 5-65, preferably, higher than about 240l/10 mm.

[0038] Said resin material should however preferably have a needlepenetration lower than about 400 l/10 mm, in order to effectively opposeto the longitudinal penetration of water along cable's interstices.

[0039] As mentioned above, the needle penetration is measured accordingto the procedure described in ASTM D 5-65, relating to the penetrationof bituminous materials. As reported in said standard method of test,the penetration of a material is the distance in tenths of a millimetrethat a standard needle penetrates vertically into a sample of thematerial under fixed conditions of temperature, load and time. Thepenetration test is thus performed according to said standard, the onlydifference being the preparation of the material. In fact, whilebituminous material are first melted, then poured into a samplecontainer and cooled under controlled conditions, the resin material ispoured in the sample container as a liquid curable composition andallowed to cure and solidify for about 24 hours, before proceeding withthe measure.

[0040] For the purposes of the present invention, in the presentdescription the term “resin” is intended to refer to a solid and compactmaterial obtainable upon curing a suitable curable polymericcomposition. Differently from water-blocking filling compositions to bedirectly contacted with optical fibers (e.g. into a buffer tubescontaining said optical fiber), which generally have a grease-like orjelly consistency, a resin filling material may better oppose to thehigh pressure water flow accidentally penetrated inside the cable.

[0041] A resin filling material having the above properties of reducedhardness can be obtained by suitably formulating a polyurethane resinprecursor with a predetermined amount of mineral oil, preferably with alimited amount of coupling agent.

[0042] The polyurethane resin precursor, i.e. a composition capable offorming a polyurethane resin upon curing, is typically apolyol/polyisocyanate mixture.

[0043] A polyurethane resin suitable as filling material according tothe present invention typically comprises less than about 35%,preferably from about 10% to about 30% by weight and more preferablyfrom about 15% to about 25%, of a polyol/polyisocyanate mixture, saidmixture being preferably comprised of from about 60% to about 85%, morepreferably from about 65% to about 80% of a polyol and from about 15% toabout 40%, preferably from about 20% to about 35% by weight of apolyisocyanate.

[0044] The polyisocyanate can be a polyisocyanate compound whichdirectly reacts with the polyol in the presence of the mineral oil andoptionally the coupling agent to form the polyurethane resin.Preferably, the polyisocyanate is a polyisocyanate prepolymer which isin turn prepared by reacting an excess of a polyisocyanate compound witha polyol in a manner well known in the art. The polyisocyanateprepolymer is then reacted with the polyol in the presence of themineral oil and the coupling agent to form the polyurethane resin.

[0045] Examples of such polyisocyanate compounds are 3-isocyanatomethyl3,5,5-trimethylcyclohexyl isocyanate (IPDI), toluene diisocyanate (TDI),4,4′-diphenylmethane diisocyanate (MDI), polymethylenepolyphenylisocyanate, 1,5-naphthalene diisocyanate, phenylenediisocyanates, 4,4′-methylene bis (cyclohexyl isocyanate) (H.sub.12MDI), hexamethylene diisocyanate (HMDI), biuret of hexamethylenediisocyanate, 2,2,4 trimethylhexamethylene diisocyanate and combinationsthereof.

[0046] Preferably, the content of urethane groups (—NCO) in thepolyisocyanate prepolymer is from about 5% to about 12% by weight,preferably from about 6 to about 10%. Suitable polyisocyanates andpolyisocyanate prepolymers are disclosed in U.S. Pat. No. 4,168,258,herein incorporated by reference.

[0047] An example of commercially available polyisocyanate prepolymer issold under the trademark UREFLEX MU 55 (Michel Baule Chimie, France),which is a solution of polyisocyanate prepolymer (65% w/w) indioctylpthalate (DOP), having an amount of about 5.5% w/w free urethanegroups with respect to the total weight of the mixture, i.e. about 8.5%w/w with respect to the weight of prepolymer.

[0048] The polyol which is reacted with the polyisocyanate compound orthe polyol which is reacted with the polyisocyanate prepolymer isselected from the group consisting of castor oil, polyether polyols,hydroxyl bearing homopolymers of dienes, hydroxyl bearing copolymers ofdienes, and combinations thereof. Although not critical to the formationof the polyurethane, the polyols generally have a number averagemolecular weight between about 1,000 and about 6,000, preferably betweenabout 1,000 and about 4,000.

[0049] Preferably, hydroxyl bearing homopolymers of dienes or hydroxylbearing copolymers of dienes are employed, hereinafter identified ashydroxylated polydienes. Said hydroxylated polydienes can be preparedfrom dienes which include unsubstituted, 2-substituted or2,3-disubstituted 1,3-dienes of up to about 12 carbon atoms. Preferably,the diene has up to about 6 carbon atoms and the substituents in the 2-and/or 3-position may be hydrogen, alkyl, generally lower alkyl, e.g.,of about 1 to about 4 carbon atoms, substituted aryl, unsubstitutedaryl, halogen, etc. Typical of such dienes are 1,3-butadiene, isoprene,chloroprene, 2-cyano-1,3-butadiene, 2,3-dimethyl-1,3,butadiene, etc. Thepreferred dienes are 1,3-butadiene and isoprene. Preferably, ahydroxylated polybutadiene is employed, having preferably a molecularweight of from about 2500 to about 3000 Dalton.

[0050] A description of suitable hydroxylated polydienes and thepreparation thereof is reported in U.S. Pat. No. 3,714,110, the contentof which is incorporated by reference.

[0051] A suitable commercial hydroxylated polybutadiene is availablefrom Sartomer Company under the trademark POLY-BD.

[0052] The polyurethane resin according to the present invention furthercomprises from about 65% to about 90% by weight, preferably from about65% to about 85% and more preferably from about 68% to about 80% byweight of a mineral oil. The Applicant has in fact found that thisrelatively high amount of mineral oil, as compared to the amount used inthe polyurethane compositions of the prior art, allows to obtain thedesired softness of the resin. In particular, the Applicant has observedthat such unusual high amount of mineral oil acts as a plasticizer agentfor the polyurethane composition, thus allowing to reduce the relativeamount of other conventional plasticizers employed in polyurethanicresin compositions (i.e. ester compounds), which may instead negativelyinteract with the other polymeric materials of the cable, as explainedin the following of this specification.

[0053] The mineral oils which may be used in the preparation of thepolyurethane filling composition of the present invention include thosealiphatic and branched aliphatic saturated hydrocarbons (both known alsoas paraffinic oils), cycloaliphatic saturated hydrocarbons (also knownas naphtenic oils and mixtures thereof, which contain from about 15 toabout 30 carbon atoms and which are distilled from petroleum. The terms“mineral oil, paraffinic oil and naphtenic oil”, as used herein, areintended in their common industrial meaning, so that said oils maycontain also some amounts (typically below 20% by weight) of aromatichydrocarbons. Preferred hydrocarbon contents of a mineral oil employablein a filling composition according to the invention are from about 35%to about 60% by weight of naphtenic oil, from about 35% to about 50% ofparaffinic oil and from about 8% to 18% of an aromatic oil. Preferably,the mineral oil employed in the resin composition according to theinvention has a content of at least about 40% by weight of a naphtenicoil. The desired properties of the composition can be advantageouslyobtained by mixing together two or more mineral oils having differentcontents and compositions of the above naphtenic, paraffinic andaromatic oils. For instance, two different mineral oils having differentviscosity (e.g. about 65 cSt and about 500 cSt) can be added in suitableproportions to the polyurethane curable composition, for adjusting theviscosity of said composition at a value suitable for the application ofthe same.

[0054] Preferably, mixtures of two or more mineral oils having aviscosity of from about 15 cSt to about 800 cSt, preferably from about50 to about 500 cSt (measured at 20° C., according to ASTM D445) areemployed.

[0055] Examples of suitable mineral oils which may be employed in acomposition according to the present invention are those produced byNynas Naphthenics (Sweden) under the commercial names NYFLEX or NYTEX.

[0056] The composition preferably further comprises less than about 12%by weight of a coupling agent, more preferably less than about 10%,particularly preferred being an amount of less than about 8% by weight.The presence of said coupling agent avoids the occurrence of possiblede-mixing phenomena which may take place in the composition beforecuring, in particular between the polyisocyanate prepolymer and themineral oil. Preferably, said coupling agent is added in an amount of atlease about 2% by weight, more preferably of at least about 4% byweight.

[0057] For the purposes of the present invention, in the presentdescription the term “coupling agent” is referred to a compound ormixture of compounds capable of avoiding the occurrence of the abovecited de-mixing phenomena. In particular, such a compound compriseswithin its molecule at least a polar group (e.g. an ester group),capable of interacting with the polar groups of a polyol andparticularly of a polyisocyanate, and at least an hydrophobic portion(e.g. an alkyl chain), capable of interacting with the hydrocarboncomponents of mineral oils.

[0058] Preferred coupling agents are esters compounds, in particular ofdicarboxylic acids, preferably di(C₂-C₁₄)alkyl esters of (C₄-C₈)alkyl oraryl dicarboxylic acids. Particularly preferred are the alkyl esters ofadipic acid or preferably o-phtalic acid, such as dioctylphtalate (DOP),dioctyladipate (DOA), diisodecylphtalate or di-2-ethylhexyladipate.

[0059] Properties and examples of these and other suitable couplingagents are reported, for instance in U.S. Pat. No. 4,168,258, which isincorporated herein by reference.

[0060] As observed by the Applicant, in the absence of the abovecoupling agent, de-mixing phenomena between mineral oil andpolyisocyanate may occur in the composition before curing, withconsequent possible non-homogeneity of the reacting system which mayresult in poor mechanical properties of the final resin.

[0061] The Applicant has however observed that the amount of suchcoupling agent should be limited below about 12% by weight, preferablybelow 10%, in order to avoid possible negative interactions of thesecompounds with the polymeric materials forming the optical core or withthe polymeric coating materials of the optical fibers.

[0062] It has in fact been observed that the presence of relatively highamounts of ester compounds may negatively affect the mechanicalproperties either of the polymeric material used for manufacturing theoptical core of the “tight” type as shown in FIG. 1 (typically made frompolyester or polyetherester polymers) or the polymeric material used forcoating the optical fibers (typically urethane-acrylate and/orepoxy-acrylate based resins).

[0063] In the specific, as observed by the Applicant, if the polyestermaterial of an optical core of the “tight” type is contacted (e.g. as aconsequence of possible accidental breaks of the thin polymeric sheath104 disposed around the optical tight core 102) with a polyurethanecurable resin composition having a relatively high amount of estercoupling agent, said coupling agent may disadvantageously permeate intothe polyester material of the optical core in an unacceptably highamount, with consequent swelling of said polymeric material. The volumeincrease of the polymeric material forming the optical core may thenresult in an attenuation of the transmitted optical signal, due to anuncontrolled compression caused by the swelled material onto the opticalfiber. For avoiding these problems, the Applicant has thus determinedthat the amount of coupling agent in the resin composition comprisingthe polyurethane components and the mineral oil shall be sufficientlylow so that the weight increase of the polymeric material forming theoptical core, upon ageing at 85° C. for two weeks in the presence of thecured resin, is preferably less than about 10%.

[0064] In addition, the Applicant has also observed that relatively highamount of ester coupling agent in the resin composition may alsonegatively affect the mechanical properties of the acrylate-basedcoating of the optical fibers, resulting in particular in a decay of theadhesion properties of the acrylate coating material onto the opticalfiber. Also for avoiding this further drawback, the amount of estercoupling agent in the resin composition shall thus be kept relativelylow, preferably below 12% by weight.

[0065] The polyurethane based filling composition according to thepresent invention may typically further contain a catalyst, in theamount of from about 5 ppm to about 200 ppm, preferably from about 5 toabout 50 ppm. Examples of suitable catalysts are tertiary amines, e.g.triethylamine, or organic salts, particularly those of tin, such asdibutyltin dilaurate.

[0066] Further additives may also be added in minor amounts to thecomposition, such as moisture scavenger defoamers, antioxidants, UVstabilizers, fungicides, biocides, flow agents or thixotropic agents.

[0067] The resin of the present invention is preferably formed byadmixing a first part (the so-called polyol component) typicallycomprising a mixture of a polyol, a mineral oil and a catalyst, with asecond part (the so-called polyisocyanate component) typicallycomprising a polyisocyanate prepolymer, a mineral oil and a couplingagent. The two components of the resin are kept separated and areadmixed only at the time of the application into cable structure.

[0068] The two components polyurethane-based filling compositionaccording to the present invention is generally applied in the liquidstate into the desired interstices of the optical cable. For instance,the resin can be applied in the liquid state onto the optical coreand/or onto the steel wires forming the armoring of the cable, whilethese latter are being stranded around the optical core during themanufacturing of the cable, in order to completely fill saidinterstices.

[0069] Once mixed, the uncured two component resin shall preferably havea viscosity relatively low so as to easily penetrate inside theinterstices to be filled with the uncured resin, but sufficiently highso as to avoid the dripping of the uncured filling composition out ofthe cable structure during the manufacturing process.

[0070] The viscosity of the curable polyurethane composition beingapplied in the interstices of the cable shall thus preferably be lowerthan about 800 cSt, more preferably lower than about 450 cSt (measuredat 25° C. according to ASTM D445, with a Cannon Feske viscometer afterabout one minute from the mixing of the components forming the resin),in order to allow an easy application of the resin in the longitudinalinterstices of the optical cable. The viscosity of the uncuredcomposition shall preferably be higher than about 150 cSt, morepreferably higher than about 250 cSt, for avoiding dripping of thecomposition during application.

[0071] In addition, the resin should preferably have a relatively highgel time, in order to allow possible interruptions in the manufacturingprocess of the cable, without incurring the risk of possible gelation ofthe resin inside the injectors of the bi-component resin. The gel timeof the resin is thus preferably higher than about 8 hours, morepreferably comprised from about 10 h and about 20 h (measured accordingto ASTM 4473). Further physical parameters which shall preferably bekept under control for a filling material according to the presentinvention are the cross-linking shrinkage, i.e. the percentage of volumeshrinkage when the liquid composition is cured into the solid resin. Arelatively low volume reduction (from the liquid to the cured resin)avoids the formation of undesired voids inside the filling material,thus creating an effective barrier of polymeric material against thelongitudinal flow of water accidentally penetrating inside the cablestructure. Preferably, said curing shrinkage is thus lower than about0.5%.

[0072] Furthermore, the polyurethane resin according to the presentinvention shall preferably evolve limited amounts of gases (such ashydrogen and carbon dioxide, for instance) upon ageing. As known, thepresence of even small amounts of hydrogen in hermetic cables may causesevere problems on the optical fibers, with unacceptable increase in theattenuation of the transmitted signal. On the other side, the amount ofcarbon dioxide is not so critical. However, if relatively high amountsof carbon dioxide are developed inside an hermetic cable, localoverpressure of the same may also cause an attenuation in thetransmitted signal, due to a mechanical compression of the opticalfibers.

[0073] A resin according to the present invention evolves less thanabout 0.6 ncm³/kg of hydrogen after aging of the resin for 15 days at100° C., more preferably less than about 0.5 ncm³/kg. In addition, aresin according to the present invention evolves less than about 150ncm³/kg of carbon dioxide after aging of the resin for 15 days at 100°C., more preferably less than about 90 ncm³/kg.

[0074] The Applicant has observed that the evolution of carbon dioxideof a resin according to the invention is relatively low with respect tocommercial resin containing rather high amounts of ester components.This is probably due to the rather high hydrophilicity of the estercompounds, which may cause a relevant absorption of water into theuncured composition, if said composition is not preserved from contactwith humidity. Said water may then react with the isocyanic groups ofthe polyisocyanate, thus evolving carbon dioxide. If substantial amountsof said evolved carbon dioxide remain trapped into the cured resin, saidgas may then be slowly released inside the hermetic structure of thecable, upon ageing of this latter, thus possibly causing undesirablelocal overpressure.

[0075] Although the cable of the invention has been described withspecific reference to a cable comprising an optical core of the tighttype, it may be appreciated by those skilled in the art that a fillingresin composition according to the invention can be advantageously usedfor filling also submarine cables comprising other types of opticalcores, such as an optical core comprising a single buffer tube centrallydisposed within said cable and loosely housing a plurality of opticalfibers therein, or an optical core comprising a plurality of buffertubes stranded around a central strength member each of said buffertubes housing a plurality of optical fibers therein.

[0076] The following examples will illustrate the invention in moredetail.

EXAMPLE 1

[0077] Preparation of a Filling Composition According to the Invention

[0078] A liquid polyurethane resin to be filled into the voids of anoptical fiber submarine cable according to the present invention hasbeen prepared as follows.

[0079] 148 g of Poly Bd R 45 HT (hydroxylated polybutadiene with anumber average molecular weight of 2,800 and a concentration of hydroxygroups of 0.83 meq/g, produced by Elf Atochem, France) were added atroom temperature to 537 g of Nytex 810 (a mineral process oil having aviscosity of 65 cSt at 20° C., commercialized by Nynas Naphtenics,Sweden), 202 g of Nytex 820 (a mineral process oil having a viscosity of500 cSt at 20° C.), 30 g of dioctylphtalate (DOP), and 0.010 g ofdibutyltin dilaurate; the mixture was stirred until homogeneity.

[0080] Thereafter, 83 g of Ureflex MU55 (Michel Baule Chimie, France),i.e. a solution of 65% w/w of a 4,4′-diphenylmethane diisocyanate (MDI)based polyisocyanate premolyer in 35% of DOP, having a total content of5.5% of free isocyanate groups, was added under stirring and the wholemixture was left to stand for 24 hours at room temperature, beforedetermining the relative softness.

EXAMPLE 2

[0081] Determination of the Softness of the Polyurethane ResinComposition

[0082] The softness of the polyurethane resin prepared according toExample 1 has been determined by the needle penetration test accordingto standard ASTM D5-65, the only difference with respect to said testbeing the preparation of the material to be tested. Thus; after theaddition of the polyisocyanate component, the liquid curable compositionof example 1 is poured in the sample container and allowed to cure andsolidify for about 24 hours. The measurement of the softness is thenaccomplished according to the above standard.

[0083] A value of 260 l/10 mm has been measured for the resin preparedaccording to Example 1.

[0084] The same test has been performed on a commercial two-componentsresin typically employed for filling the interstices in submarineoptical cables, having an amount of plasticizer agent (palmitate ester)higher than 50% w/w (about 70%). A value of 110 l/10 mm has beenmeasured for this commercial resin.

EXAMPLE 3

[0085] Compatibility of the Resin Filling Material with the PolymericMaterial of the Cable Core

[0086] In order to evaluate the compatibility of the liquid polyurethanecomposition with the materials forming the optical core, test pieces ofthe material forming the optical core were immersed into polyurethaneliquid compositions with different amounts of DOP coupling agent, whichcompositions were then cured and aged.

[0087] Hytrel 3548 L (a thermoplastic elastomer polyetherester sold byDU PONT) was used as the reference material forming the tight opticalcore, for producing specimens of 2 m length and 2.76 mm diameter.

[0088] The weight of each specimen has been measured and the specimenswere then immersed into different liquid polyurethane based compositionswhich were then cured. The tested polyurethane compositions werecomprised of 17.5 parts by weight of polyurethane resin precursors (witha polyol/polyisocyanate ratio as in Example 1), 82.5 parts of mineraloil (Nytex 810), 10 ppm of dibutyltin laurate as catalyst and DOP ascoupling agent in variable relative amounts, as indicated in table 1.The specimens were thus immersed into the liquid resin composition,which has been cured and aged for 15 days at 85° C.

[0089] At the end of the test, the specimens were extracted from theresin and their weight was determined.

[0090] The following table 1 lists the result of these tests. TABLE 1Weight increase of polymeric optical core upon ageing at 85° C. for twoweeks Weight increase MO:DOP ratio PU:MO:DOP ratio % 97:3  17.5:8.0:2.57 93:7  17.5:77:5.5 8 87:13 17.5:72:10.5 10 73:27 17.5:60:22.5 14 68:3217.5:56:26.5 16 48:52 17.5:40:42.5 22 24:76 17.5:20:62.5 30

[0091] As shown by the above table, an amount DOP coupling agentslightly higher than about 10% may cause an increase of about 10% in theweight of the polymeric material forming the optical core.

EXAMPLE 4

[0092] Compatibility of the Coating Acrylate Material of Optical Fiberswith the Polyurethane Resin Composition

[0093] An optical core of the “tight” type, having a diameter of 2.76 mmand containing four optical fibers has been manufactured, using Hytrel3548L as the polymeric material for embedding the optical fibers. Theoptical fibers were commercial LEA fibers with acrylate coating.

[0094] Four specimens of the above optical core (two meter length each)were immersed into a polyurethane liquid compositions prepared accordingto Example 1 and four into specimen were immersed into the commercialpolyurethane resin of Example 2.

[0095] The resins have been cured and aged at 85° C. into a ventilatedoven for one month.

[0096] At the end of the ageing, the fibers were extracted from theoptical core and the pull out force of the acrylate coating of eachfiber was determined.

[0097] As a reference, the pull out force has been measured also on theoptical fibers embedded into four different specimens of non-aged tightoptical core.

[0098] The pull out force of the acrylate coating of the optical fibersis measured as follow.

[0099] One end of the optical fiber is adhesively fixed to a movableclamp and the other end of the fiber is fixed to fixed clamp connectedto a load cell. The acrylate coating is cut along its wholecircumference (for a thickness corresponding to the thickness of theacrylate coating) in the proximity of the movable clamp, to create adiscontinuity in the coating. The movable clamp is then moved away fromthe fixed clamp at a constant speed of about 1 cm/min and the pull outforce (expressed in g/cm) for separating the acrylate coating from theglass portion of the fiber is measured.

[0100] The following tables 2a-2c show the results of the test made onthe four specimens in the three different conditions. TABLE 2a Pull outforce of optical fiber in a non-aged optical core Pull out force (g/cm)Fibre Spec. 1 Spec. 2 Spec. 3 Spec. 4 Average Orange 1260 1225 1325 12981275 Brown 945 900 855 1060 940 Yellow 1170 1105 1160 1090 1131 Black800 1020 950 1040 952

[0101] TABLE 2b Pull out force of optical fiber in an optical core agedin the presence of the polyurethane composition of Example 1 Pull outforce (g/cm) Fibre Spec. 1 Spec. 2 Spec. 3 Spec. 4 Average Orange 850910 1075 1195 1010 Brown 905 1065 980 1100 1012 Yellow 925 990 910 1070973 Black 785 810 1020 1035 912

[0102] TABLE 2c Pull out force of optical fibers in an optical core agedin the presence of a commercial polyurethane composition Pull out force(g/cm) Fibre Spec. 1 Spec. 2 Spec. 3 Spec. 4 Average Orange 750 400 335630 528 Brown 735 700 200 450 521 Yellow 865 650 540 515 642 Black 530415 480 585 502

[0103] As apparent from the above tables, the average value of the pullout force of optical fibers in an optical core aged in the presence of apolyurethane composition according to the present invention issubstantially the same as the pull out force of non-aged optical fibers.On the other side, the pull out force of optical fibers in an opticalcore aged in the presence of a commercial polyurethane fillingcomposition are clearly lower, thus indicating possible negative impactsof this kind of filling compositions on the mechanical properties of theoptical fibers.

1. Optical cable comprising: an outer sheath; an optical core disposedwithin said outer sheath, said optical core comprising at least oneoptical fiber housed therein; at least one longitudinal cavity disposedalong and in contact with said optical core; wherein said at least onelongitudinal cavity is at least partially filled with a resin materialhaving a needle penetration higher than about 200 l/10 mm, measuredaccording to ASTM D 5-65.
 2. Optical cable according to claim 1 whereinsaid resin material has a needle penetration higher than about 240 l/10mm.
 3. Optical cable according to claim 1 wherein said resin is apolyurethane-based resin.
 4. Optical cable according to claim 1 whereinsaid optical core comprises at least one optical fiber embedded into amatrix of polymeric material.
 5. Optical cable comprising: an outersheath; an optical core disposed within said outer sheath, said opticalcore comprising at least one optical fiber housed therein; at least onelongitudinal cavity disposed along and in contact with said opticalcore; wherein said at least one longitudinal cavity is at leastpartially filled with a polyurethane resin obtainable from a liquidcurable composition comprising less than about 35% by weight of apolyol/polyisocyanate mixture and from about 65% to about 90% by weightof a mineral oil.
 6. Optical cable according to claim 5 wherein theamount of the polyol/polyisocyanate mixture in said liquid curablecomposition is from about 10% to about 30% by weight,
 7. Optical cableaccording to claim 5 wherein the amount of the polyol/polyisocyanatemixture in said liquid curable composition is from about 15% to about25%.
 8. Optical cable according to claim 6 wherein saidpolyol/polyisocyanate mixture comprises from about 60% to about 85% of apolyol and from about 15% to about 40% by weight of a polyisocyanate. 9.Optical cable according to claim 6 wherein said polyol/polyisocyanatemixture comprises from about 65% to about 80% of a polyol and from about20% to about 35% by weight of a polyisocyanate.
 10. Optical cableaccording to claim 5 wherein said amount of mineral oil in said liquidcurable composition is from about 65% to about 85%.
 11. Optical cableaccording to claim 5 wherein said amount of mineral oil in said liquidcurable composition is from about 68% to about 80% by weight. 12.Optical cable according to claim 5 wherein said liquid curablecomposition further comprises less than about 12% by weight of acoupling agent.
 13. Optical cable according to claim 5 wherein saidliquid curable composition further comprises less than about 12% byweight of a coupling agent.
 14. Optical cable according to claim 12 or13 wherein said coupling agent is an ester of a dicarboxylic acid.